Thread cutting tool



Feb. 11, E936. R THOMSON `2,029,514 T THREAD CUTTING TOOL FiledApril 25, 1934 INVENTOR ATTORN EYS Patented Feb. 4, 1936 v 'A I Y.

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THREAD CUTTING TOOL Robert Thomson, Arlington, N. J., assigner to Dardelet Threadlock Corporation, New York, N. Y., a corporation of Delaware [application April 25, 1934, Serial No. 722,28? 1 claim. (o1. iii-14.1)

FFICE An important object of the present invention is From the entering end of the tap, inward to provide a tap of improved design for cutting along the turns of the helix included in the Dardelet internal screw threads. length a, the teeth occur in uninterrupted heli- Another object of the invention is to provide cal sequence on the successive lands. These a tap to cut a complete Dardelet thread in a sinleading or pilot teeth are designated 6. Be-

gle operation and having thread groove cutters tween the teeth 5 the groove 5 is of materially and thread crest trimmers in Yan improved arless Width than the teeth. Along the following rangement and correlation to reduce working length of the tap up to the crest trimmers l, the

strain upon the tap and to enable a quite small teeth occur in interrupted helical sequence, a

Dardelet thread to be cut with accuracy in retooth being omitted from every alternate land 10 fractory materials having a tendency to bind and around the tap and along the helix. Consequent- Cil clog the tap. ly along this medial length of the tap the groove Other objects of the invention will appear is much wider. The interrupted teeth are desighereinafter. nated 8. The crest trimmers 'l follow the teeth 3;, In the drawing, Fig. l is a side View of the E. They are in the form of wide shallow teeth 15 improved tap in one of its forms; having straight cutting edges at the top, inclined Fig. 2 an enlarged transverse sectional View slightly with reference to the tap axis. Preferapproximately on the line 2--2 of Fig. l; ably the inclination is six degrees. Each land has Fig. 3 a side View of the improved tap in a one crest trimmer and the trimmers are arranged modied form; and in a helical path parallel with that of the two 20 i a fragmentary longitudinal sectional series of groove cutting teeth 6 and 8. They are view on an enlarged scale, showing a Dardelet formed by grinding down each land on a six deinternal thread and a Dardelet external thread gree taper with an angled grinding wheel of sufriin mutual locking engagement. cient thickness to span the width of two adjacent The tap illustrated in Figs. l and 2 is fluted to groove cutting teeth. The grinding is continued '2 provide preferably three lands, numbered l, 2 until the medial, active portions of teeth l exand 3. At its entering end the tap tapers to a tend aClOSS a theOleCal pXOlOlgatiOn 0f the point. From said end taper the tap is tapered for bottom line of groove 5.

a length a. lt is also tapered to a less degree The crest trimmers may increase progressively 3.0 along a following length "b. The number of in depth, or, the third trimmer, or both the sec- I tapered lengths and the degree of taper may be ond and third trimmers may be of the same depth varied, however. The tap has the usual cylinas the rst trimmer to smooth and burnish the drical shank li. third crest and to function in practice, upon dull- Upon the tap lands there are thread groove ing or mutilation of the preceding trimmer, to

cutting teeth, thread crest trimmers and thread nish the trimming operation. 35 finishing and guiding teeth all in an improved Following the crest trimmers are the tap guidarrangement and correlation. To io-rm the teeth, ling teeth 9 which may also serve to finish the a helical groove 5 is cut in the tap along the en thread groove if the groove has not been finished tire working length of the latter. Except for the by the teeth preceding the crest trimmers. The

points where the crest trimmers occur on the teeth 9 occur in uninterrupted sequence on the 40 lands, the bottom of the groove is straight in lands in succession and they lie on a prolongation cross section and is parallel with and equidisof the helical path oi the teeth 6 and 8. All of tant from the tap axis at all points. The groove the teeth 6, l, 8, and 9 may be formed With a cutting teeth and the nishing and guiding teeth suitable top relief.

are all of the same width at the base or root. A pair of mutually self-locked Dardelet threads 45 The depth of the cutting teeth, however, inis shown in section in Fig. 4. The internal and creases progressively along the taper lengths a external threads are designated i and il reand b to gradually increase the depth of the spectively. The crest surface of the thread lil thread groove cut by the tap. Following the is inclined to the axial line at an angle of six length b all of the groove finishing and guide degrees to form the self-holding binding surface 50 teeth on the lands are of the same depth and i2 which Wedges upon a similarly inclined bottom their top edges are straight in cross section and surface i3 of the bolt thread groove, upon screwparallel with and equidistant from the axis of the tightening rotation of one thread on the other tap. The side faces of al1 the teeth converge Without axial advance, as more fully explained outward slightly. in the United States patent to Dardelet No. 55

1,657,244. It is important that the crest size of the rib of the thread l actually cut by a tap shall conform accurately with the standard which the tap is designed to cut, that the angle of the crest surface shall be accurate and that the crest surface shall be smooth.

Heretofore it has been difficult and often impossible to tap small Dardelet threads in copper, open hearth steel, or materials having the characteristics of wrought iron. Such materials tend to contract and bind the tap and also to clog it. A slender tap for tapping very small holes would frequently break in two under the resistance offered by such metals. Or, the teeth of the tap and the thread being cut would often become mutilated so that the thread would be unfit to cooperate with a complementary thread for self-locking.

The improved tap has been designed to tap small holes in said refractory materials without dangerous strain upon the tap. The tap is employed to tap a hole whose diameter slightly eX- ceeds that of the helix on which the bottom of the tap groove 5 lies, so that no cutting will be performed by the edges of the groove bottom on the lands. The leading end of the tap is inserted into the hole and the uninterrupted pilot teeth 6 accurately center the tap with relation to the hole and perform the initial groove cutting. As the tap advances relatively through the hole the depth of the groove is gradually increased and the groove cutters 8 come into action. Due to the interrupted sequence of the cutters 8 the working contact of the tap with the hole is cut in half and increased clearance is afforded for escape of chips. Consequently binding and clogging strain upon the tap is greatly reduced and breaking of the tap and mutilation of its teeth is prevented. The groove cutting teeth preserve their correct prole so that they are able to produce an accurate thread rib for presentation to the crest trimmers which immediately follow the groove cutters. The thread rib arrives at the crest trimmers in an unmutilated state so that an effective locking surface of full width can be produced by the trimmers.

The tap shown in Fig. 5 is designed more especially for tapping somewhat larger holes. This tap has a blunt leading end and a single taper which is suflcient to eliminate some of the uninterrupted pilot teeth 6. Also, this tap is designed for a more balanced support of the tap to hold it accurately centered at opposite sides of the crest trimmers 1. To this end the interrupted sequence of the groove cutting teeth terminates short of the crest trimmers and several teeth 8nu occur close together in uninterrupted sequence directly in advance of the crest trimmers. Directly following the crest trimmers there are also several of the guiding teeth 9 in uninterrupted sequence or close together to cooperate with the teeth 8a in centering and supporting the tap at opposite sides of the crest trimming operation. The following teeth 9a occur in interrupted sequence to reduce frictional contact of the tap with the work. Except for the modications described this tap is like the one rst described.

What I claim is:

A tap for cutting Dardelet internal screw threads, characterized in that said tap has a body threaded and Fluted to provide lands with a helical series of thread groove cutting teeth in interrupted sequence on the lands in succession around the tap to reduce strain one the tap, a. helical series of thread groove cutting teeth in uninterrupted sequence immediately following said interrupted teeth, a helical series of thread crest trimmers immediately following said uninterrupted teeth and having cutting edges inclined slightly toward the tap axis longitudinally of the tap, the land surfaces at and between the bases of all or the groove cutting teeth on the lands in advance of said crest trimmers being nearer the tap axis than seid crest-trimming edges to avoid all crest-trimming action in advance of said crest trimmers, a helical series of grooveentering guide teeth in uninterrupted sequence immediately following said crest trimmers, and a helical series of groove-entering guide teeth in interrupted sequence immediately following said uninterrupted guide teeth.

ROBERT THOMSON, 

